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Anyone who went to both conventions, racing from the shores of Lake Erie to the banks of the Delaware with only a couple of days in between, may be suffering from ideological and tonal whiplash.

The Republicans convened last week in a country under siege, imperiled by terrorists, illegal immigrants, criminals and most ominously by Hillary Clinton. Donald Trump in Cleveland said he'd be the law-and-order president, and then said it again, and then said it a third time on the off chance anyone missed the concept.

The Democrats gathered here this week in a still-great, still-hopeful country, but one in desperate need of more love, empathy, justice and equality. That softer tone has emphasised diversity, togetherness and caring. Keynoter Cory Booker used the word "love" 10 times on Tuesday. The New Jersey senator said, "We are called to be a nation of love".

Today's rhetoric was poised to be tougher as the party turned to national security, with President Barack Obama and Vice-President Joe Biden scheduled to take the stage in prime time. But for the first two days, the Democrats assiduously positioned themselves as the nurturing party.

The contrasting tones carry political risks for each party in what has been a violent and chaotic northern summer, with terrorist attacks at home and abroad. The Republicans are the party of change this time, led by an insurgent outsider who has never held elected office and says America in a "moment of crisis". The Democratic establishment, fully in command here if not always in control, is banking that the public isn't eager for a Trump-scale disruption of American political life.

The strategists want to send a message to the public: We hear you. We know how you feel. Beyond the presidential race, there is a battle for both houses of Congress and countless state and local contests; these conventions are golden opportunities for the parties to make an emotional connection with voters.

Michelle Obama used the word "kids" or "children" or referenced her own daughters or the role of parents 39 times in her much-lauded prime-time speech on Tuesday.

One of the most effective speeches came yesterday, when Lauren Manning, who suffered burns on 82 per cent of her body during the 9/11 terrorist attack in New York City, spoke of being visited in her hospital room by then-Senator Clinton: "She took my bandaged hand into her own".

"Certainly fear is a strong motivator, but I think hope and love is a better plan," said Wisconsin delegate Karla Stoebig, 33, an aspiring veterinarian who stood in her delegation on the convention floor.

"Our platform is inclusive to all kinds of people, without the divisive, hate-filled rhetoric we saw at the Republican convention," said Crystal Miller, 35, a beauty adviser in Wisconsin - and, like Stoebig, wearing a "cheesehead" hat.

Walking past City Hall earlier in the day, Carl Levin, the retired six-term senator from Michigan, and his brother Sandy, a 17-term congressman, paused on the pavement to ponder the difference in tone between the conventions.

"His speech was the worst speech in terms of tone," Carl Levin said of Trump. "It's obviously an ego speech and I think that's not going to work for him."

"There is an autocratic streak in Donald Trump," Sandy said.

"And that has never appealed to Americans," Carl said.

"Instead of love, it's narcissism," Sandy said.

Fear vs hope may be a simplistic and coarse framing of the presidential contest, but the candidates and their strategists have made that unavoidable. Some traditional issues, such as free trade, are topsy-turvy this year, with Trump taking a position to the left of Clinton. There's been little talk of taxes. Even fractious social issues like gay marriage and abortion haven't gotten much attention. Instead there's a sharp contrast in tone.

Some of the differences between the two conventions have been structural and geographical. The Republicans met in an arena right downtown. The smaller dimensions of Cleveland, the natural perimeters created by the Cuyahoga River and Lake Erie, and the decision by locals to take the whole week off meant that downtown Cleveland became a kind of GOP bathtub, all partisans and journalists and law enforcement officers with hardly a regular 9-to-5 worker in sight.

The Democrats, however, have soaked into the fibre of the much larger city of Philadelphia in a more subtle fashion. The big hotels are full of delegates, and the plazas around City Hall sprout protests reliably on the hour, but the convention proper is a long, hot ride by subway, taxi or Uber down Broad Street, in an arena surrounded by parking lots and media tents.

Republican presidential nominee Donald Trump, on the final day of the Republican National Convention in Cleveland. Photo / AP

Republican presidential nominee Donald Trump, on the final day of the Republican National Convention in Cleveland. Photo / AP

Another obvious difference is that all the Democrats showed up here, showing off the party's diversity and attendant divisions. Many of the most distinguished Republicans, including the two most recent Republican presidents, skipped Cleveland, unwilling to associate themselves with Trump. The result was a convention comparatively heavy on testimonials from Trump's wife and children.

One common feature of both conventions: organisational missteps and unscheduled dissent. The Republicans had a plagiarism kerfuffle and then a stiff middle finger from Ted Cruz in prime time. The Democrats had their hacked email scandal, the deposing of the party chairman and then a steady series of disruptions and even a walkout yesterday from Bernie Sanders supporters.

The Sanders partisans marched into one of the media tents, taping shut their mouths to signal their sense of being silenced. Others chanted "The whole world is watching!" in homage to the famous chant among protesters at the riotous Democratic convention in Chicago in 1968.

One of the "Bernie or Bust" protesters, Werner Lange, a 69-year-old retired teacher and a Sanders delegate from Ohio, said he doubted Sanders had endorsed Clinton of his own volition: "They must have gotten to him. My impression is that they threatened his family."

A hot debate soon broke out in a concourse within the arena, with the Rev Antonio Anderson, of Hope Church Philly, telling Sanders partisans that they need to work to get Clinton elected. The Sanders folks vehemently disagreed, saying that she's no better than Trump and is uninspiring.

"Where are the Hillary marches?" one young woman asked.

"Hillary don't need to march," Anderson replied, and pointed to the interior of the arena where the pro-Clinton speeches were ongoing. "They're in there!"

Party conventions are extra-constitutional; there is nothing in the historic document written in this city in the summer of 1787 that countenances such a thing as a Democrat or a Republican, much less a nominating convention with 15,000 credentialed members of the news media and associated protests and policing. So every convention is a staged event, more or less scripted, with improvisations and surprises and big moments that no one quite saw coming.

David Axelrod, the Democratic strategist who now is a talking head for CNN, cautioned against premature story lines coming out of the second half of July.

"Conventions are like four-act plays," Axelrod said yesterday, "and you can only measure them when the play is done."

"In Cleveland we heard a message of 'Be Afraid, Be Very Afraid,' " said Virginia Senator Mark Warner as he navigated a crowded concourse at the Wells Fargo Centre. He said the more positive message from Democrats was not something carefully orchestrated by party leaders: "We're Democrats; we'd never be that organised."

Democrats said in numerous interviews that they think their sunnier message will resonate with voters in November.

But this is shaping up as another highly competitive White House contest. One Democratic operative who has worked on a number of campaigns, and did not want to be identified because of the sensitivity of the issue, was sombre as he rode the subway to the arena.

"I believe love is stronger than hate, but fear wins elections," he said.